Add streams

[ddbeck]

This is only partly finished. Questions abound:

• Are streams, in the abstract, a feature? Caniuse has one for it, so I assume yes, but maybe I'm wrong.
• Is BYOB a core part of this? Caniuse says yes but it seems like developers are using streams without this.
• Is api.WritableStreamDefaultController.signal a distinct thing, like abortable fetch?

[foolip]

Matching https://caniuse.com/streams fully here doesn't feel like an option, but I think we can decide which notes we consider for our "initial support" feature.

I would say that note 2 (Only basic read support) and 4 (No support for WritableStream) do warrant partial implementation, because just reading or just writing means that streams is being used as part of some other feature, I can't see how it could be used standalone without at least write and write support.

BYOB support however I think we should consider not a part of the initial feature.

All in all, I'd expect these versions to fall out, assuming caniuse data is correct:

• Chrome 59
• Firefox 100
• Safari 14.1

[ddbeck]

OK, got it. We can't exactly match that with BCD (I suspect because caniuse doesn't note the missing WritableStream interface in Safari before 14.5), but we can get pretty close. See the latest diff.

[foolip]

Thanks @ddbeck! Merged!